1. Field of the Invention
The present invention relates to a holographic memory medium on which information is recorded by fixing interference fringes generated when a signal beam interferes with a reference beam therein, a holographic memory device for recording and reproducing the information in and from the holographic memory medium, and a holographic recording device for recording the information in the holographic memory medium. In particular, the present invention is suitable to correct a tilt error caused between the holographic memory medium and the reference beam.
2. Description of the Related Art
Generally, in the holographic memory, information is recorded by fixing an interference fringe, which is generated by an interference caused therein between a signal beam and a reference beam, in a holographic memory material layer. In this case, the signal beam is subjected to spatial light modulation according to the information to be recorded. Therefore, when the signal beam and the reference beam are applied to the holographic memory, a bright and dark interference fringes are generated in the holographic memory material layer according to the information to be recorded. A highly polymeric monomer in the holographic memory material layer is drawn to a “bright” area of the interference fringes to be polymerized, thereby fixing a refractive index distribution in the holographic memory material layer corresponding to the interference fringe. As a result, information is recorded on the holographic memory.
It is known that in the holographic memory, plural kinds of information can be simultaneously recorded in one recording area (recording block) by changing an incident angle of the reference beam with respect to the holographic memory material layer (angular multiplexing). In other words, the reference beam made incident at different angle for each different piece of information to spatially modulate the signal beam, to thereby make it possible to fix interference fringes corresponding to each different piece of information independently of one another according to their incident angles on the same recording area.
During reproduction, the reference beam is applied to the holographic memory material layer at the same angle at which the reference beam is applied during the recording. Thus, diffraction occurs in the reference beam according to the interference fringe of the angle, and the diffracted reference beam is received by a photoreceptor element to reproduce the information recorded at the angle.
JP 11-16374 A and JP 2000-338846 A each describe a holographic memory device based on angular multiplexing.
In a case of recording information by angular multiplexing, generally, an incident angle of a reference beam with respect to a holographic material layer is changed in an in-plane direction of a surface including the optical axes of the signal beam and the reference beams. Thus, even when a tilt error occurs between the holographic memory and the reference beam in the in-plane direction during reproduction, the incident angle of the reference beam with respect to the holographic memory can be adjusted to a proper state through controlling an actuator (galvano mirror or the like) for adjusting the reference beam according to the tilt error.
For example, as shown in FIG. 12A, when an interference fringe is generated in the holographic memory material layer, even if a tilt occurs in a plane along the line X-Y in FIG. 12A in the holographic memory during reproduction, the reference beam actuator is driven and controlled to correct the incident angle of the reference beam as shown in FIG. 12B, to thereby make the reference beam to be incident on the holographic memory at a proper angle.
However, when the tilt error is caused in a direction perpendicular to the surface including the optical axes of the signal beam and the reference beam, that is, when the tile error is caused in an in-plane direction of the X-Z plane shown in FIGS. 12A and 12B, the tilt error cannot be corrected by drive control of the actuator for reference beam. In such a case, an angle between the interference fringe and the reference beam is different from that at the time of recording, so it is impossible to obtain an adequate reproduction signal.